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Report on the Geology & Gold Fields of Otago
The Auriferous Reefs
The Auriferous Reefs.
General Geological Observations.—As introductory to the description of the reefs, the following remarks on the nature of the country rocks in which the reefs occur, or gold has been found in matrix, will save frequent repetitions. With the exception of those of the auriferous locality near Portobello, specially considered further on, the rocks consist throughout according to my observations, and which are confirmed by Captain Hutton’s, the Provincial Geologist’s more extensive and detailed geological survey—of metamorphic schist, i.e., argillaceous mica schist or phyllite, changing from the east towards the west into real mica schist, with subordinate bands of chlorite-schist, or chloritic mica schist, The line of page 157boundary where this change takes place has with difficulty been traced by Captain Hutton, but it appears that within the large district on the west, occupied by mica schist, there exists at least one rather large area, viz., the Carrick Range, where the rock conforms in all lithological respects to true phyllite, and this range lies, according to Captain Hutton, within the line of a main anticlinal axis. Whether this inlying patch of phyllite represents a remnant of denudation of an once superincumbent general formation or the mica schist, or whether it simply constitutes an area where the metamorphic action and change were of less intensity, remains a difficult problem, to be solved by future investigation. Only at two places within the extensive metamorphic district I obtained evidence of the existence of an intrusive rock, viz., high up the Carrick Range, in small dyke-and knoblike protrusions of a dark "hornstone-porphyry," and at Alexandra in several specimens of a similar porphyry, said to be derived from a reef-like out-crop (no doubt a dyke) on the northern slope of the Old Man Range. Mr. Coleman, of Alexandra, knows of several more dykes of this kind in other localities. Massive occurrences of granite, such as characterise the neighbourhood of most of the Victorian Gold Fields are, however, quite absent.
As a general rule, both the phyllite and mica schist, but more especially the latter, are rich in interlaminations of quartz, generally from less than one-fourth to near one inch in thickness, but sometimes assuming considerable dimensions—one to three feet in thickness—though with no regularity and permanency in strike and dip.
These generally lenticular-shaped masses, and, as a specimen shown me at Alexandra proved, also the small interlaminations of quartz, have at several places been found auriferous, which led to their being erroneously mistaken for true reefs or their leaders; and a considerable amount of money and labour has been wasted in their exploration. As a case in point. I may here notice the workings on the so-called Butcher’s Gully Reef, near Alexandra. There, on a rocky, high table-land, consisting of nearly flat bedded mica schist, full of quartz interlaminations, a fine large shaft has been sunk 60 to 70 feet deep, and furnished with a pump worked by a waterwheel, which received its water supply by means of a long wooden flume from an extensive race on the neighbouring range. Gold at the rate of 6 dwt. per ton is said to have been crushed from the quartz, but as neither in the shaft nor in the stuff worked out of it, nor in the surrounding country, any indications of a reef are visible, the auriferous quartz found must no doubt have been derived from one or several of the small interlaminations, and the prospects of the place would, therefore, certainly not warrant further expense in more extensive exploration There are certain inferences to which the existence of these auriferous quartz interlaminations lead, namely, that they may be more frequent and more widely distributed page 158than hitherto supposed, and that the riches in gold of the drifts of the Province are in some measure, at least, due to their denudation. As regards the strike and dip of the schists, they are subject to great changes throughout the country, the former running through nearly all directions of the compass, and the latter from horizontal to vertical, from one side to the other; and these variations are in some places, as for instance on the Carrick Range, so frequent, and exist in such close proximity to each other as to render the taking of any mean dip and strike quite a hopeless task. I have therefore, in the subsequent descriptions of the reefs, been in most instances obliged to give merely the relative position of the latter to the country—i.e., whether they traverse the rocks either in strike or in dip, or in both. On the large scale there exists, as already mentioned, according to Captain Hutton’s observations, a main anticlinal axis running nearly north and south through the country, including the Carrick Range in its course, from which axis the average dip of the rocks is on the one hand eastward, on the other westward; and in the latter direction, more especially between Arrow and Skipper’s Creek, a more than usual regularity obtains in the westward dip, as seen in the ranges bounding the Arrow and Shotover Rivers.
Grouping of the Reefs.—The auriferous reefs opened throughout the Province differ very much, both in structure and mode of development; still there are some districts of which, though they lie rather fur apart, the reefs show much resemblance to each other in the above respects; whilst again in other districts, comparatively close together, the difference in the nature and behaviour of the reefs is very great indeed. Taking advantage of these alliances, in order to simplify the description, the reefs and other occurrences of gold in matrix may be grouped as follows:—
| 1st | Group: The Saddle Hill Reef, Green Island, near Dunedin; the Reefs of Tokomairiro (Canada Reef, &c.); the Gabriel’s Gully Reef, near Lawrence; the O. P. Q. Reef, Waipori. |
| 2nd. | Group: The Reefs of Bendigo, near Cromwell; the Rough-Ridge Reefs; Conroy’s Gully Reef, near Alexandra. |
| 3rd. | Group: The Reefs of Carrick Range. |
| 4th. | Group: The Reefs of Arrow and Skipper’s Creek. |
| 5th. | Group: The Reefs of Macrae’s Flat and Shag Valley. |
| 6th. | Group: Exceptional occurrences of gold in matrix—the so-called Peninsula Quartz Reef at Portobello. |
| 1st. | Group: Saddle Hill Reef, Gabriel’s Gully, O. P. Q. Reef, Canada Reef.—These reef are true lodes, promising permanency in depth. They have well defined foot and hanging walls and clay casings or selvages, and cross the country (phyllite) both in strike and dip, though the difference in angle, either in strike or dip, or page 159sometimes in both, is generally not considerable. In their structure, development, and mode of occurrence they resemble very closely a certain class of Victorian reefs, called "Block Reefs," the typical characteristic of which is, that they are composed of generally a number of blocks of quartz, either with contractions of the lode fissures between, or, what is the case here, alternating with blocks of mullock—the latter term meaning gangue matter, consisting of country rock slipped into the lode fissure, where in course of time, it became more or less mineralised, impregnated with pyrites and traversed by small quartz veins. These quartz and mullock blocks, which reach sometimes considerable dimensions, extend hardly ever vertically downward, but show an endlong dip in strike within the fissures— north or south, east or west, as the case may be—in the same reef invariably, in the reefs of the same district generally, in the same direction. The feature may, in fact, be considered as an oblique banded structure on the large scale. In the reefs under notice the thickness of the blocks reaches in some cases 12 feet, and their dip in strike is generally at rather flat angles. According to experience, gold occurs both in the quartz and mullock blocks; but the former have hitherto invariably been found the richest, and in them its distribution is mostly not uniform throughout, but, as found in the Gabriel’s Gully Reef, for instance, a rich small shoot dips at a flat angle across the block; or, as in the Canada Reef, a number of narrow, rich runs, dipping in the line of the blocks, are separated by poorer quartz; or, as in the Waipori Reef, a certain thickness on the hanging and foot walls is richer than the centre, &c. The appearance of the quartz, and this appertains also to that of the reefs of the other groups, varies very much according to whether the stone comes from near the surface or from beneath the water-level. In the first case it is white, opaque, and mottled and striped, with brown iron ore, or ferruginous slaty matter; in the latter it shows a bluish color, is slightly translucent and glassy, and full of blackish spots and seams of slaty matter, which, as well as the quartz itself, are more or less densely impregnated with pyrites. With the exception of the O.P.Q. Reef, Waipori, which produced from the old workings a very good average per ton, and promises richer yields from the new ones, the other reefs of this group are rather poor in gold, the yield ranging not much above 5 dwts. per ton throughout. Still, considering the thickness and extent of the quartz-blocks, the facility with which they can generally be worked, and that sometimes the intervening mullock is worth crushing, on account of thin rich quartz seams traversing it, they ought to be profitable to work—i.e., on a larger scale than has hitherto been the case—for at least several hundred feet in depth; more especially if attention is paid to the saving and treatment of the pyrites; and the truth of this opinion is in some measure already shown by the results of the Canada Reef Mine, which, though worked by shaft on page 160a limited scale, produces from a depth of 80 feet, according to the manager’s statement, a small profit from an average yield of 5 dwts. of gold per ton. |
| 2nd |
Group: The Reefs of Bendigo, and the Rough Ridge, Conroy’s Gully Reef near Alexandra.—My reasons for grouping these reefs together are based more upon their exhibiting unmistakable structural differences from the reefs of the other groups than upon any similarity to each other in several aspects, touching mode of development and relation to the country. Their thickness is, in the average, but small, ranging generally from one half to two feet, and exceeding rarely four feet and, as far as workings have proved, they do not consist of solid quartz throughout, and but seldom so for any considerable extent in strike and dip (Logan’s Reef). They represent, in tact, in certain respects, "block reefs," though with this difference from the true reefs of this class, that blocks of quartz and mullock of irregular size and outline are more or less irregularly intermixed, and do not, as those of the latter do, dip at certain angles, and in the same direction in strike. Most of the reefs of Bendigo show well-defined walls with clay casings, strike nearly uniformly east and west, are mostly traceable for long distances, and—what constitutes them very "strong" ones in mining sense, and indicates permanency in depth—they traverse horizontally, or very flat-bedded mica schist, vertically, or at very steep angles. The reefs of the Rough Ridge vary in strike, though not at large angles, and most are not traceable far in strike. They are generally not so well defined as those of Bendigo, and seem liable to frequent irregularities in strike and dip, contractions, and more especially to being faulted by slides; but these unfavourable features are apparently the results of surface disturbance only, and may disappear in depth. It is not uncommon, both at Bendigo and the Rough Ridge, that reefs split in strike into branches, which, though deviating at first from, assume gradually again the strike of the main reef, and run thus pretty close and parallel together, some increasing to the same, or even a greater thickness than that shown by the latter. Besides "leaders" that dip towards them, so-called "droppers," dipping at generally flat angles away from them, have also been observed in some of the reefs at both places. With regard to the mode of occurrence of the gold, the comparatively superficial work done on most of the reefs hardly permits to form a definite and generally applicable opinion. Judging, however, from those most extensively and deepest worked, it would seem that the metal is accumulated in rich shoots of variable width, that dip at rather steep angles in strike in the quartz blocks; whilst the portions intervening between the shoots, including the mullock patches, are poor, but generally rich enough, or of such limited extent as to render it the most economical to work them with the rest, without resorting to special selection. Excepting the yields from Logan’s page 161Reef, as unusually rich ones, those from the other reefs have been by no means low in the average, as they ranged up to 2 oz. per ton, and from hardly a single reef were they reported much below half an ounce per ton. Considering this, I was much astonished to see so many of the reefs and claims neglected (at the Rough Ridge over 20 claims were once worked, whilst at present only a single one); but the reason was explained to me to consist partly in the high prices charged for crushing, partly in expensive cartage, and more especially in the want of enterprising miners, those who worked the top having become afraid of the hard work and expense required in contending with the water. With regard to the prospects of the reefs in depth, I consider them, where the reefs are well defined, as very favourable, both as concerns persistency in auriferous character and regularity in average size. But in speculating upon profits to be derived from future working, several important points must not be left unconsidered, viz., the comparatively small size of the reefs, expenses connected with getting rid of the water, and greater difficulty in extracting the gold from the quartz; for below the water level, which lies in several of the reefs considerably higher than one would suppose from their elevation above the nearest permanent surface water, the seamy quartz is throughout very metalliferous; in fact, it promises at greater depth to become more highly charged with metallic sulphides (iron and arsenal pyrites, galena, zincblende, &c.) than in any of the reefs of the other districts I examined. Timely attention to improvements in the gold-saving appliances is therefore highly advisable. The expense of working narrow reefs will in depth also considerably increase, in consequence of the increasing hardness and closeness of the country, which latter is comparatively more unfavorable to work on account of its horizontal bedding. However, against this may be placed as a perhaps more than adequate set-off the high dip of the reefs, which much facilitates working, and, what is of greater importance, that the hardness of the rock, in conjunction with its horizontal bedding, renders the supporting of the workings very cheap and simple. |
| 3rd |
Group: The Reefs of the Carrick Range.—The generality of these reefs present in several respects quite distinctive characters from the reefs of the other groups. They are peculiar clayey ferruginous "mullock reefs" or rather "quartz-mullock reefs," so soft that they can mostly be worked by pick without the aid of boring and blasting; and the quartz, which apparently forms no large per-centage of their mass, occurs only in the shape of coarse sand, and small angular and slightly rounded pieces—such reaching or surpassing the size of a fist being rather rare. Whether it represented originally interlaminations in the mullock, or was formed in veins, is uncertain, but a kind of banded structure in the line of dip of the reefs speaks in favour of the latter. These reefs vary in page 162thickness from less than 1 foot to over 6 feet; they strike in all directions across the country, but are only of short extent, and differ very much both in direction and angle of underlay—the latter ranging from vertical to less than 20°. Some of the reefs show also much irregularity in their course, for they expand and contract, twist and curve in strike and dip in quite a peculiar manner, and are—what is the case also with most of the others—frequently faulted by slides and cross-courses, so that it requires very great attention and perseverance on the part of the mining managers not to lose them. Considering all these points in connection with the fact that the country—a rather soft phyllite—is also very much disturbed both in strike and dip—steep and flat dips alternating and changing in direction within short distances—it appears next to certain that not only the peculiar soft and gravelly nature of the reefs, but also the exceptionally flat dips of some are not original, but due to strong pressure, friction, upheaval, etc.; and as the cause of these disturbances appears the most likely the intrusion of the dark hornstone-porphyry, which, as mentioned at another place, occurs in small knobs and dykes at several places on the range (near Carricktown.) Unfavourable as these features no doubt appear, touching straightforward and uninterrupted working of the reefs in future, I feel no apprenension of the latter giving out suddenly, or at a limited depth, for they are in every respect true lodes, crossing the country both in strike and dip, and showing the most frequently the hanging wall, less frequently the foot wall, and in some instances both walls well defined and separated from their mass by clayey casings, mostly polished and striated, representing the so-called "Slicken-sides," which afford unmistakable proof of movements of the wall of the reefs. The gold, both in the quartz and mullock, is very fine, and, owing to the sott and ferruginous nature of the stuff specks can but very rarely be seen during working. Judging from the crushings and occasional washing of prospects, it occurs chiefly in shoots dipping in strike, less in irregular patches, but seems also to be pretty generally distributed throughout the whole extent of the reefs, as far as opened. The yields of most of the reefs opened have in the average been very fair, as they ranged between ½ and 1½oz. per ton. On account of its softness the quartz mullock is easily crushed, but the saving of the fine gold requires great attention; and, as the supply of water, which the proper treatment of this kind of stuff requires, is rather above the average, but has at the existing machines been frequently much below it, and their saving appliances are not the most suitable ones besides, I am sure a great deal of the gold has been lost in the tailings. There is at the level of even the lowest workings not much pyrites observable in the reefs, still the ferruginous character of the mullock. as being no doubt a result of its decomposition, points to its former presence in large r quantities page 163and it may with certainty be expected to increase in abundance in depth. As regards the expense of working the reefs, the soft nature of both the mullock and country renders it small in one respect, viz.:—that of exploitation proper; in another, however, viz:—that of supporting the workings rather large, on account of the high price of timber, and the expense in this respect increases of course, in order to avoid accidents and collapse, the flatter the dip of the reefs. Fortunately there are on the field experienced managers and miners, well able to cope with this difficulty in the most economic and practical manner. |
| 4th | Group: The Reefs of Arrow and Skipper’s Creek.—The only reefs in this group in course of being worked, and of which I was able to examine the workings, are "Southberg’s" and the "Nugget and Cornish." Skipper’s Creek; still, from examination of the outcrops of some reefs at Arrow, and information received about the character of a number of others once worked, but since abandoned, in both districts, I was enabled to form the following opinion on the general character of the group:—These reefs are true massive lodes, ranging from 4 to over 20 feet in thickness, which cut through the country both in strike and dip—the latter being generally steep—and show more or less well defined walls, with clay casings; a number are traceable for long distances – some for miles—in strike. In point of composition and structure they approach, however, far more mullock reefs than quartz reefs—they represent, in fact, fissures partly filled with debris from the country, full of interlaminated quartz, partly occupied by bunches and veins, of variable size, of true reef quartz. The mullock seems in the larger reefs to be generally predominating, and forms in places where their width very much increases by far the greater part of their mass—in fact, experience tends to prove that the thicker a reef is, or the wider it becomes the more mullock it contains, whilst on the contrary, decreasing thickness is connected with a relative increase in quartz, and the reefs become also better defined. The bunches and veins of reef quartz occur either on the hanging or footwalls, or on both walls, rarely in the centre. They appear to dip shootiike in strike, and are generally only in payable quantity within the line of the quartz shoots, or mineralized mullock, with its interlaminations and fine cross veins of quartz, has also, in all the reefs opened, been found to contain gold throughout, though generally only in payable quantity within the line of the quarts shoots, or where the reefs much contract in size. The yields have ranged from several dwts. to over 4oz. of gold per ton, but average from the reefs at present worked about 10 to 16 dwts. per ton. Although none of the reefs at Skipper’s Creek have as yet been opened below permanent water level, they are already highly charged—both quartz and mullock—with pyrites, which seriously interferes with the satisfactory saving of the generally fine gold during crushing. This led the Phœnix page 164Company, after Mr F. Evans, the manager, had proved the payably auriferous character of the ore as such, to erect in connection with their crushing mill the necessary works for extracting the gold from the large quantity of it saved on long blanket strakes. The country is, with regard to most reefs, very favourable for their being easily and cheaply worked, owing to the highly precipitous nature of the mountains which they traverse, and the deep valleys and gorges from which they can be opened, either directly in strike, or by but short cross adits. Still there is at Skipper’s Creek one serious drawBack to regular systematic working, affecting the most important of the reefs within certain depths, namely, although the country rock—mica schist—appears on the large scale but little disturbed in strike and dip, it is throughout highly fissile, and traversed by numerous cracks and joints, and these features combined, aided by percolation and freezing of water, have originated enormous slips from the precipitous mountain sides—faults, in a certain sense—by which the continuity of the reefs is completely broken. In fact, nowhere on the slopes of those steep mountains can the miner be certain of having a reef exactly in its original place or unshifted. These disturbances, which render working dangerous and cause much expense in timbering, extend, however, to within the level of the nearest gorges or valleys only; below this, there is every probability of the lay of the reefs becoming less broken, though the existence of smaller faults and other irregularities must there also be apprehended, on account of the fissured nature of the country. As regards the continuance of the gold in depth, I see no reason to doubt it, yet I think it very likely that the doubtless increasing abundance of pyrites may, as in Victoria—and this refers also to the reefs of all the other groups—be connected with a corresponding decrease of fine, free gold in depth. |
| 5th |
Group: The Reefs of Macrae’s Flat and Shag Valley.—In point of definition and mode of occurrence these reefs, which I found all deserted, are, in my opinion, the least promising ones of the Province. Judging from those I could examine, and information about a few others that have been prospected, they represent either so called "layer-lodes," (Duke of Edinburgh Reef, Macrae’s Flat, Shag Valley Freehold Company Reef), or are merely interlaminations between the beds of phyllite that forms the country in both districts. Touching the layer-lodes, their general characteristics are that they strike and dip with the country, having the foot wall (one and the same bed of the country) generally pretty smooth throughout; but the hanging wall, mostly quite irregular, uneven and traversed by leaders. They are, on account of this mode of relation to the country, subject to all the changes in strike and dip of the latter, and, if these are great, are liable to frequent changes in thickness and con generally not be depended on for persistency in depth. In page 165the cases under notice, the stratification of the country is fortunately pretty regular, and all points are therefore in favour of the reefs being also more regular in course and thickness, and having a better chance of persistency in depth than is usually the case. Their dip is rather flat (less than 45[unclear: 2]), and they are from 2 to 5 feet thick in the average, being composed of mullock and rather good looking quartz; the latter generally predominating and occurring in bunches and veins mostly on the foot wall, or being irregularly intermixed with the mullock. The gold occurs both in the quartz and the mullock, though mainly in the former, and the yields have ranged from a few dwts. up to 2 oz. per ton, but did not pay in the average; therefore the reefs were abandoned at a very shallow depth. Considering that the gold did not run out, and that the reefs, as far as opened, show a good thickness for considerable extents, in connection with the circumstances under which they were worked, the advisability of giving them another more extensive and systematic trial certainly deserves consideration. With regard to the interlaminations, which were considered to be reefs, and of which some are in places several feet thick, they conform in all respects to what was said about these bodies in the beginning. That they are quite unreliable, concerning gold-bearing character and extent in strike and dip, is proved by the fact that, though several carried good gold (above 1 oz, per ton) near the surface, either their irregularity or impoverishment, or both cases combined, rendered working soon unpayable, and they were deserted in consequence, and, I may remark, do not invite renewed and more extensive prospecting. |
| 6th | Group: Exceptional occurrence of gold in matrix.—The so-called Peninsula Quartz Reef at Portobello.—These curious occurences of gold which I inspected, in company with Captain Hutton, the Provincial Geologist, do not, though believed to do so, represent a quartz reef at all, but are, as far as the superficial workings and the reported results of trials allow one to judge, impregnations of gold in a finely divided state through various kinds of rock that will be described farther on. In a geological point of view, this auriferous locality and neighbourhood are the most interesting I have seen during my visit, and would well deserve a special detailed topographical and geological survey. True trachyte, trachytebreccia, and tufa, and indurated ash-beds, broken through by dykes, and irregular dyke-like masses of basalt, compose principally, as it seems, that part of the Peninsula; whilst sedimentary rocks—sandstone and limestone—the geological relation of which requires yet to be determined, form apparently a narrow strip between it and the mountain on which Mr. Larnach’s mansion is built. This mountain, consisting in its upper part of trachyte and basalt intermixed, seems, from half-way down its slope, towards the base, to be composed of sandstone, which in a small quarry is clearly seen to dip underneath the volcanic top rock.* As regards the places page 166opened—four in number, in three of which gold has been proved by trial crushings to exist—they lie pretty nearly in a line (the supposed line of reef) down the steep slope of a mountain, at that part composed of greyish-white trachyte. Progressing from above downward, the place highest up the slope consists of an old sawpit, from which a short prospecting drive has been put into the mountain. Quartz of a rather concretionary character was here found in the shape of an irregular bunch, enclosed with trachyte—the whole quantity lying about amounting to about one ton. In this no gold was seen, and no trial was made of it, but a specimen has been reserved for assay. The second opening, about 100ft. lower down hill, is a small, shallow excavation in trachyte, which is here full of siliceous segregations of irregular outline—some nearly one foot in diameter—of a quartzite-like character and bluish white colour ("bastard quartz," in miners’ phrase), in which very fine grains of pyrites can be seen in abundance. Of this stuff two trial crushings, one of 2 cwt., gave 18 grains; the second, of a ton, yielded ½ oz. of gold; and Messrs. Forbes and McAuley, the prospectors, who kindly showed us over the ground, affirmed that they could wash a pretty fair prospect of fine gold from every tin dishful of the small stuff excavated. In the third opening—a small open cutting, some 60ft. below the previous one—very close-grained white trachyte was excavated, of which a trial crushing of a ton yielded 3 dwts. of gold. The fourth and last opening lies at the foot of the mountain, and consists of a good shaft of 40ft. in depth, with a small drive at the bottom. It penetrated for the first 25ft. through loose ground, and the last 15ft. through a decomposed cap into a hard coarsely crystalline rock, composed of hornblende, triclinic feldspar, and some quartz, and being more or less densely impregnated with very fine grains of pyrites. This rock continued also in the drive. Three trials were made, viz:—Two of the hard rock of 1 ton and ½ ton, which yielded respectively 8 dwts. and 11 dwts. of gold; and the third of 1 ton of the softer decomposed cap-rock, which gave, strangely enough, only 6 dwts. of gold, whilst generally decomposed portions of pyrites-bearing rocks are richer than undecomposed ones. All the recorded trial crushings were executed at a good battery in Victoria, and, in order to remove further doubt about the genuineness of this strange auriferous character of the quartzose, white trachyte, and the rock from the shaft, several small samples were most carefully tried in Dunedin, and these all produced gold. Accepting the auriferous character of the rocks, therefore, as satisfactorily proved, and considering that between the places opened there exists virtually not a feature such as would indicate a narrow auriferous zone or streak in their line down the mountain side, whilst it would seem very improbable that just per accident the auriferous portions of the rocks were opened and exhausted, we must come to the conclusion that there is a page 167great likelihood of tie gold being generally disseminated—richer and poorer in places—through these peculiar varieties of rock as far as they extend. As the matter stands, the average results of the washings from Nos. 2 and 4 workings are certainly such that, taking into account the facilities the ground offers for mining, abundance of timber close to, etc., they should render working on a large scale, with ample crushing machinery near at hand, highly payable. Considering this, it would be really deplorable if the still lingering doubts in the reliability of the results of the trials made, i.e., whether the gold really came out of the stuff and not out of the crushing machine, were not definitely set at rest by a further and more extensive trial—say of 10 to 15 tons from each of the two good places. |
Strange as the occurrence of gold in such matrix, and under such circumstances, no doubt appears, it is in reality not without its alliances—at least, in certain respects—both in another part of New Zealand and in foreign countries. Captain Hutton, who is intimately acquainted with the Thames Gold Field, North Island, recognised at once a certain resemblance between the geological features of the locality under notice and those of the Thames district. He thought the greyish white trachyte of the former looked much like the gold-bearing trachyte-tufa of the Thames, though there, as well known, the gold is found in bunches and veins of genuine quartz, and does not occur finely disseminated through the mass of the rock. The hard crystalline rock of No. 4 workings he also considered similar to a rock of which dykes traverse the auriferous tufa of the Thames, but which itself has not been found auriferous. As regards my own experience touching this rock, I think the latter bears considerable resemblance to certain trachytic rocks (trachyte greenstone) which I have seen on a journey through Hungary and from Transylvania, and which are there rich in auriferous silver and lead lodes. But from comparative examinations of specimens, I can also state that in mineral composition it is quite identical with, and in appearance hardly distinguishable from, the quartzose diorite-greenstone of some of the dykes which in Victoria traverse upper Silurian rocks, and are themselves traversed by generally highly auriferous quartz veins. That the rock is, notwithstanding this latter close resemblance, of volcanic origin, and represents in reality "trachyte greenstone," there can hardly be a doubt, however, on account of its mode of association with the typical trachyte of the locality. For besides in the shaft, close round which the greyish white trachyte is plainly exposed, outcrops of it (the greenstone) have also been found at several places in a neighbouring gully which cuts through trachyte, and on the slope of the trachyte range. According well with this opinion about its origin, and what it lithologically represents, and further showing the importance of this kind of rock, is what the celebrated page 168geologist Von Richthofen reports from the Washoe country and other parts of America, namely, that the rock there most prolific in gold and silver lodes is a volcanic rock of the trachyte series, closely allied to diorite both in composition and appearance, and for which he proposes the name "Hornblende propylite." Considering all the foregoing observations in connection, there exists, besides what we already know of their auriferous character as such, the chance that any quartz reefs or veins found traversing the trachyte and trachyte-greenstone in the locality under notice, or, in fact, wherever they occur, may prove highly auriferous. And on this account not only the neighbourhood of the workings, but the whole of that part of the Peninsula is well worth a thorough prospecting. The shore line showing the rocks generally plainly exposed offering in this respect special advantages.
* See Anti Page 48.